A typical hammer drill comprises a body in which is mounted an electric motor and a hammer mechanism. A tool holder is mounted on the front of the body which holds a cutting tool, such as a drill bit or a chisel. The hammer mechanism typically comprises a slideable ram reciprocatingly driven by a piston, the piston being reciprocatingly driven by the motor via a set of gears and a crank mechanism or wobble bearing. The ram repeatedly strikes the end of the cutting tool via a beat piece. When the only action on the tool bit is the repetitive striking of its end by the beat piece, the hammer drill is operating in a hammer only mode.
Certain types of hammer drill also comprise a rotary drive mechanism which enables the tool holder to rotatingly drive the cutting tool held within the tool holder. This can be in addition to the repetitive striking of the end of the cutting tool by the beat piece (in which case, the hammer drill is operating in a hammer and drill mode) or as an alternative to the repetitive striking of the end of the cutting tool by the beat piece (in which case, the hammer drill is operating in a drill only mode).
EP1157788 discloses a typical hammer drill.
Hammer drills are supported by the operator using handles. In one type of hammer drill, there is one rear handle attached to the rear of the body of the hammer drill, at the opposite end of the body to where the tool holder is mounted. The operator pushes the cutting tool into a work piece by pushing the rear handle towards the body, which in turn pushes the body and the cutting tool towards the work piece.
A problem associated with hammer drills is the vibration generated by the operation of the hammer drill, and in particular, the vibration generated by the operation of the hammer mechanism. This vibration is transferred to the hands of the operator holding the handles of the hammer drill, particularly through the rear handle. This can result in the injury of the hands of the operator. As such, it is desirable to minimise the effect of vibration experienced by the hands of the operator. This is achieved by reducing the amount by which the handle vibrates.
There are at least two ways of reducing the amount by which the rear handle vibrates. The first method is to reduce the amount of vibration produced by the whole hammer drill. The second method is to reduce the amount of vibration transferred from the body of the hammer drill to the rear handle. The present invention relates to the second method.
EP1529603 discloses a dampening mechanism for a rear handle by which the amount of vibration transferred from the body to the handle is reduced.
The rear handle is slideably mounted on the body using connectors 230. Springs 220 bias the handle 202 rearwardly away from the housing 212, and which act to dampen vibration to reduce the amount transferred from the housing 212 to the handle 202. A movement co-ordination mechanism is provided, which comprises an axial 216, which interacts with the connectors 230 to ensure that the movement of the two ends of the handle are in unison.
The problem with the design of dampening mechanism disclosed in EP1529603 is that the movement co-ordination mechanism is located within the housing. As such, it takes up valuable space.
EP2018938 seeks to overcome this problem by placing the movement co-ordination mechanism in the handle.
However, in both EP1529603 and EP2018938, the designs of handle require a movement co-ordination mechanism which incurs extra cost and complexity.
In EP152603, there are provided two bars (230a, 230b) connected to the handle which slide within guides (232a, 232b) mounted on the housing. In EP2018938, there are provided two bars (24; 104) connected to the housing which slide within guides (26) mounted on the handle. In both designs, the amount of contact in the lengthwise direction between the bars and the guides remain constant at all times. The amount of contact is dependent on the length of the guide. This is regardless of the position of the handle versus the housing. As such, the amount of support for the bars against a bending force applied to the bars remains constant regardless of the amount of force applied to the handle to move it towards the housing. Only the position of the guides on the bars alters as the handle moves relative to the housing.
Furthermore, the guides are shown as making contact along the whole length of the part of the bars located inside of the guides. However, in reality, the inner surfaces of the guide and the external surfaces formed on the bar are not perfectly flat due to manufacturing tolerances and wear. Therefore, to ensure that the bars slide smoothly within the guides, the dimensions of the cross section of the bars are slightly less than that of the cross section of the passageways formed through the guides. This however, allows the bars to move by a small amount in a direction perpendicular to its longitudinal axis within the guide. This allows the handle to move sideways thus increasing the amount of vibration transferred to the handle.
EP 2289669 discloses a hammer drill in which a rear handle is moveably mounted on to the rear of a body via at least one movement control mechanism and which is capable of moving towards or away from the body, wherein each movement control mechanism comprises a first mount, a rod, having a longitudinal axis, rigidly connected at one of it ends to the first mount, and a second mount which slidingly engages with the rod at two distinct points only along its length to allow the rod to slide relative to the second mount in a direction parallel to the longitudinal axis whilst preventing the rod from moving relative to second mount in a direction perpendicular to longitudinal axis, wherein one mount is attached to the body and the other mount is attached to the rear handle.
As there are only two distinct points of contact, there is no contact between the rod and the second mount any where else. It will be appreciated that at each of the two points where they slidingly connect, a part of the second mount can slide along a part of the rod or that part of the rod can slide along a part of the second mount.
The use of two distinct points of contact only ensures that a good contact can be made with the rod at theses points in order to provide a strong sideways support for the rod against a bending force acting on the rod, thus preventing any sideways movement of the rod.